Robot wrist mechanisms are carried at the end of mechanical arms of robotic manipulators, which are used to position and move parts and tools, generally termed end effectors.
In most robot manipulators so far proposed, the end effector is mounted on the wrist mechanism which consists of a series of links each of which can pivot about an axis mounted on the next link so that the end effector can be turned about any or all of three different, usually orthogonal, directional axes.
In a first known form of wrist, each successive link carries an axis at 90.degree. to its own axis. In a second form of wrist, each successive link carries an axis at 60.degree. (or thereabouts) to its own axis. To a greater or lesser extent, both types of mechanism exhibit degenerate positions in which the three axes of rotation become co-planar. In these positions, the end effector can only be turned freely about two directional axes instead of the usual three. A particular feature of these mechanisms is that when the wrist links are close to a degenerate position, that is, when the axes are nearly coplanar, the rate at which the end effector can be rotated is restricted because the rate of rotation of the wrist axes required to obtain end effector rotation is much greater is such a position than in other positions. Thus, there exists a zone about each degenerate position, usually called a "singularity zone", in which one or more of the rotary motions of the end effector is restricted in speed to a much greater extent than in other positions of the wrist mechanism.
If one were to show the range of orientation movement of the wrist on a sphere, one would find zones of degeneracy or singularity zones in which part of the end effector motion is restricted. Most wrist mechanisms so far proposed exhibit one or more singularity zones as described within the range of orientation movement permitted by mechanical limitations. Thus singularity zones further restrict the usefulness of a wrist mechanism.
A practical wrist mechanism will also be restricted in the extent to which each rotation can occur because of the impossibility of passing mechanical parts through one another, and the need to provide a degree of strength and stiffness in the mechanism and thereby increasing the size of supporting members.
A further requirement of a practical wrist is that the hydraulic and electrical service and signal lines (or umbilicals) for the end effector must be positioned so as not to interfere with the actuation of the wrist and not to become tangled when the wrist is manipulated through its full range of angular movements. While it is sometimes possible to carry the umbilicals through the wrist axes, where the end effector has substantial power and feedback signal requirements, as in the case of an automated sheep shearing mechanism, this will not be achievable without compromising the size, mass and dexterity of the wrist. Thus in many wrist applications, the end effector services must be carried by an external umbilical system which itself must remain under control when subjected to high dynamic forces and must also be excluded from the work space.